This invention relates to a light emitting device comprising a UV or blue light emitting diode or laser diode (LED) and an excitable phosphor. More specifically, the present invention relates to a phosphor coated LED having a specific geometry disclosed for the coating designed to improve the efficiency of the LED.
There is currently a market for LED's for general illumination, so called “white LED's”. These “white LED's” emit radiation that appears substantially white to those that perceive it. The most popular white LED's consist of blue emitting GaInN epitaxially grown layers on sapphire (single crystal alumina) or single crystal SiC. The blue emitting chips are coated with a phosphor that converts some of the blue radiation to a complimentary color, e.g. a yellow-green emission. Together the blue and yellow-green emission produces a white light typically with a correlated color temperature of about 5000K and a color rendition index, Ra, equal to about 70-75. There are also white LED's which utilize a UV emitting chip and phosphors designed to convert the UV radiation to visible light. Typically, two or more phosphor emission bands are required.
White phosphor coated LED's typically have package efficiencies of about 50-70%. The package efficiency is defined as the ratio of the actual light output of the LED to the light that would be obtained if all the radiation generated escaped from the package without being absorbed. In the invention described herein, package efficiencies approaching 100% can be realized.
Historically, phosphor coated LED's have rather low package efficiencies partly because phosphor particles generate light that is radiated equally in all directions. Some of this light invariably is directed toward the LED chip, substrate, submount, and lead structure. All these elements absorb some of this light. In addition because the phosphors typically are not perfect absorbers of long wavelength UV or blue radiation some of the initial excited radiation emitted by the LED chip itself is also reflected back onto the aforementioned structural elements. Finally in the case of UV emitting chips, in order to absorb all the UV and avoid UV bleed through, the phosphor coating must typically be relatively thick, at least 5-7 particles thick. This further increases the coating's visible reflectance. The light lost due to absorption of radiation by the LED chip, submount, reflector and lead structure limits the package efficiency.
As mentioned, typical package efficiencies are 50-70%. Hence there is a significant opportunity for improving the efficiency of LED packages if the package efficiency could be increased to near 100%. Fluorescent lamps, for example, which also utilize phosphor coatings, have package efficiencies close to 100% mainly because the light which is generated by the phosphor coating and radiated back into the lamp does not strike any absorbing structures.
Another major problem that is addressed by the present invention is phosphor coating uniformity. Current designs leading to the above-mentioned package efficiencies typically have the blue or UV emitting chip mounted on a substrate and then placed in a silver coated reflector cup. The cup is filled with a silicone or silicone epoxy with the phosphor powder embedded in it. Phosphor particles are distributed randomly in the silicone slurry, which, in addition to the above-mentioned effect of reduced package brightness due to scattering light back, the relative phosphor thickness also differs greatly over the geometry of the coating. This results in color separation in the beam pattern. It also leads to different colors for different parts due to different coating patterns and thicknesses as well as undesirable blue or yellow rings in the LED emission pattern.
The problem of phosphor coating uniformity has been addressed in U.S. Pat. No. 5,959,316, in which a uniformly thick fluorescent or phosphor layer is separated from an LED chip by a transparent spacer. The entire assembly is then embedded in a transparent encapsulation epoxy resin.
Another problem that is encountered in conventional LED packages is that the efficiency of the phosphor is decreased when it is positioned in a layer on top of or adjacent the LED chip. This is due to the residual heat of the chip warming the phosphor and changing its emission characteristics. Still another drawback to conventional LED packages is that, due to the fact that the phosphor coating is applied non-uniformly, the total amount of phosphor used is often more than is necessary for the efficient conversion of the light emitted by the chip. Phosphor compositions are relatively expensive and this additional amount increases the total cost of the LED significantly.
One way to minimize light losses in LED's is to insure that the submount, reflector and lead structure are coated with as large amount of reflecting material as possible. Most manufacturers practice this approach. Nevertheless, the LED chip itself, especially in the case of a chip with a SiC substrate, absorbs significant amounts of both its own radiation and that of the phosphor radiation. Further, other parts of the LED structure, for example the submount, are rather strongly absorbing of visible and near UV radiation. Surprisingly, even silver coated reflector and lead structure elements are somewhat absorbing of both of these radiations. Due to this absorption and the fact that so much of the radiation bounces between the phosphor coating and the LED structure, package efficiencies exceeding 50-70% are rarely realized even with coated surfaces.
One alternate approach to putting the phosphor in the silicone in a reflector cup is practiced in LumiLED's LUXEON™ LED products. In these designs, the emitting LED chip is coated with a thin conformal coating of phosphor. This arrangement reduces non-uniformity in the thickness of the coating over the chip as well as promoting LED to LED color uniformity. However, it may actually decrease the overall efficiency of the LED because the chip and submount are absorbing and more than half the radiation generated by the phosphor coating is reflected directly back onto these components.
Therefore, it would be advantageous to design a phosphor coated LED having a maximum light output by increasing the package efficiency of the LED to above 70%, and preferably close to 100%.
Further, it would be desirable to produce UV/phosphor or blue/phosphor white LED's with a uniform phosphor layer and consistent color throughput and, in the case of UV emitting chips, an LED without significant amount of UV radiation leakage to the environment.
It is further desirable to increase the efficiency of the phosphor conversion by applying a uniform coating thickness of the phosphor and positioning this coating away from the LED chip to prevent heat from the chip from being transmitted thereto.
In addition, it is desirable to minimize color shift of the LED due to current fluctuations. A color shift with current is often observed in phosphor coated LED's due to the high radiation flux density on the phosphor, which tends to saturate the phosphor by depleting the ground state of certain activators. In the invention described herein, by remotely coating the phosphor the blue flux density (W) from the LED chip is greatly decreased.